2 BTC for the example of Bitcore to be maintained. That's quite a lot for a fee and it's not that actually far from the fee that other exchanges are asking, of course except Binance and other the same tier exchanges.
I also thought of that they make profit from their internal exchange and their transaction fees, but asking developers to maintain their listing is something new to me, they can do that on low level coin but not on coins like Bitcoin and Ethereum and some of the top coins I will never think of using a wallet not supporting popular and top coins in the market, imagine getting a multi wallet wallet without Ethereum there.